1. Field of the Invention
The present invention relates to telecommunications methods, apparatus, and systems. The present invention more particularly relates to mode assignment and mode adaptation to channel conditions in wireless telecommunications systems utilizing single carrier or multicarrier data transmission.
2. State of the Art
Wireless telecommunication and data transmission systems, as a rule, utilize several operational modes, depending on radio channel quality. For example, for data transmission rates between 6 Mbits/s and 54 Mbits/s the American IEEE 802.11a WLAN standard specifies eight different modes, while the European HiperLAN2 WLAN standard specifies seven different modes. The modes are specified in Table 1:
TABLE 1Data RateMode(Mbits/s)ModulationCoding Rate#802.11aHiperLAN802.11aHiperLAN802.11aHiperLAN166BPSKBPSK1/21/2299BPSKBPSK3/43/431212QPSKQPSK1/21/241818QPSKQPSK3/43/45242716-QAM16-QAM1/2 9/166363616-QAM16-QAM3/43/47485464-QAM64-QAM2/33/4854—64-QAM—3/4—
From Table 1 it can be seen that the various modes differ in both modulation techniques and encoding parameters. Therefore, the proper mode should be set up in a transmitter and in a receiver in advance for each communication session. However, the standards do not specify a procedure of mode setting up or mode adaptation to channel conditions.
The IEEE 802.11a standard indicates that it is the responsibility of the MAC (Media Access Control) layer to assign the proper mode. The only signal parameter mentioned in the IEEE 802.11a standard which can be used for mode assignment and mode adaptation to channel conditions is the RSSI—Receive Signal Strength Indicator (see, IEEE 802.11a, Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specification in the 5 GHZ Band, Sections 17.2, 17.3, 17.5). According to the IEEE standard, the RSSI is a measure of the RF energy received by the OFDM PHY. In particular, the RSSI is a measure of the energy observed at the antenna used to receive the current PPDU (PPDU is the PHY signal, including preamble, header, data, tail and pad bits). The RSSI is measured during the 16 μs PLCP preamble (PLCP—physical layer convergence procedure), and the RSSI is a monotonically increasing function of the received power. RSSI indications of up to eight bits are supported.
The IEEE standard does not indicate how MAC and PHY should use the RSSI; but it says that RSSI may be used as a part of the CCA (Clear Channel Assessment) scheme. CCA is determined in the IEEE standard as a PHY mechanism, which shall detect a medium busy condition and report the result to the MAC. The CCA indication has two states—BUSY and CLEAR, and the MAC considers this indication before issuing the permission for signal transmission. The IEEE standard also explains that a receive level more than −82 dBm shall cause CCA to indicate the BUSY state with a probability >90% within 4 μs of the preamble. If the preamble was missed, the receiver should hold the carrier sense (CS) signal busy for any signals above −62 dBm.
Apparently, the RSSI is a sufficiently reliable parameter for clear channel detection through the CCA mechanism. However, it is not reliable parameter for mode assignment and mode adaptation. The main disadvantage of RSSI as a basis for mode assignment and for the adaptation procedure is that the RSSI is an ambiguous function of the signal-to-noise ratio (SNR) or signal-to-interference ratio (SIR) which determines real performance of the system.
The HiperLAN2 standard also does not specify a procedure for determining the mode. However, it does indicate that the system uses a link adaptation scheme; i.e., based on link quality measurements, the PHY layer mode is adapted to the current link quality. This link adaptation is in both downlink and uplink (each HiperLAN2 frame includes fields for both downlink and uplink transmissions).
The adaptation procedure for the HiperLAN2 standard can be described as follows. An access point (AP) receiver measures the uplink quality and determines which mode the transmitter of the Mobile Terminal (MT) shall use for uplink data transmission (it being noted that the algorithm of making that determination is not specified). The AP sends the corresponding request to the MT within the Frame Channel (FCH), which is a fixed field in the MAC frame. In a similar way, the MT measures the downlink quality and signals a mode suggestion to the AP transmitter. The MT then sends the corresponding request to the AP within the Resource Request (RR) field.
It will be appreciated by those skilled in the art that the adaptation procedure in the HiperLAN2 standard is based on a measurement of the link quality, but the algorithm and measured parameters are not defined. So, the problem of channel quality measurement is still unsolved in the existing standards. Meanwhile, optimization of mode assignment and mode adaptation to channel conditions remains an important issue of wireless system design.